What does 285-30R21 mean?
Tire sizes are easy to understand once you learn how to read them. On this page you'll find a color-coded explanation of the 285-30R21 tire code.
This number indicates that your tire has a width of 285 millimeters.
This number means that your tire has an aspect ratio of 30%. In other words, your tire's sidewall height (from the edge of the rim to the tire's tread) is 30% of the width. In this case, the sidewall height works out to be 85 millimeters.
This letter denotes how your tire was constructed. Radial is the standard construction method for about 99% of all tires sold today.
The tire size was designed to fit rims or wheels that are 21 inches in diameter.